Forests and Water: A State of the Art Review for Colorado

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tion are simply lost to increased evaporation from the soil (e.g., Bates and Henry, 1928; Troendle, 1987a). (The word "saved" is in quotation marks because on a global scale water is generally not created or lost, and the concept of "saved" water is only valid for the area under discussion.) A 1982 review of paired watershed experi- ments noted that annual precipitation must exceed 450- 500 mm (18 -20 inches) in order to detect an increase in runoff as a result of removing much of the vegetative cover (Figure 2.2) (Bosch and Hewlett, 1982). As annual precipitation increases beyond 450 -500 mm, vegetation density increases and there is a corresponding shift from soil evaporation to interception and transpira- tion as the dominant sources of ET or water "loss." ET can be predicted for the higher elevation forests in the Fraser Experimental Forests by equation 2: ET = 460 mm + 0.28 (P — 460 mm) (2) where P is the annual precipitation in millimeters (Tro- endle and Reuss, 1993). This equation indicates that un- 700 600 E 500 N Q W 400 9 W Y K rW 300 a 3 200 100 til annual precipitation exceeds 460 mm (18 inches), all of the precipitation is used for ET. Twenty -eight percent of all precipitation beyond this threshold will be lost to interception, and the balance becomes runoff. These principles mean that vegetation removal will re- sult in progressively larger increases in annual runoff as precipitation increases. Reviews of paired catchment experiments in Colorado, the U.S. (Stednick, 1996), and throughout the world (Bosch and Hewlett, 1982) show that this general trend is maintained up to at least 1600 mm (approximately 60 inches) of annual precipitation (Figure 2.2). Table 2.1 summarizes the initial increases in annual water yields that have been observed from paired- watershed studies in or particularly relevant to Colorado. The observed increases in annual water yields range from approximately one inch at Wagon Wheel Gap to 3.6 inches in the Upper Basin at Deadhorse Creek in the FER Larger increases are generally associated with greater amounts of precipitation, higher pre - treatment water yields, and removal of a greater proportion of the forest cover. 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 MEAN ANNUAL PRECIPITATION (W Figure 2.2. First -year increases in water yield after vegetation removal versus mean annual precipitation. Note that 500 mm is approximately 20 inches (from Bosch and Hewlett, 1982).